The present invention is concerned with debugging code which has been compiled using an optimizing compiler.
Code is generally written in a high level programming language. This high level language, often referred to as source code, is translated by a compiler program into an assembly language. The binary form of the assembly language, called object code, is the form of the code actually executed by a computer.
Code debuggers are programs which aid a programmer in finding errors in code. They are extremely useful tools for improving the efficiency of the code debugging process. Many code debuggers supply information pertaining to operation of the code on the assembly code level. If the original code is written in a higher level language, however, this makes program debugging a difficult operation. When a programmer writes his program in a high level language, he does not want to search for the appearance of these errors in the assembly code.
To avoid this problem, it is desirable to develop debugger programs which allow a programmer to debug his program with reference to the level of code in which he originally wrote the program. Such a debugger program is often called a source-level debugger.
One of the important features of a code debugger is to allow a programmer to stop the execution of code and to check the values in each user resource the code is operating upon. A user resource is typically a variable defined in the source code. The values in the user resources give clues to indicate the source of trouble when a program is not operating correctly.
Since the computer operates on object code, a source level debugger needs to know where user resources named in the source code are actually stored by the computer during operation, so that when a user requests the current value for a user resource, the debugging program knows where to find the user resource. Typically, a compiler will allocate a storage location for the user resource where the user resource is always stored. In this case, the debugger need simply go to the location and access the value of user resource.
Difficulties arise when compilers are used which generate optimized code. Usually the design goal of an optimizer within a compiler, is to generate code that executes as fast as possible. In optimized code it may be desirable not to store a user resource in the same place all the time. For instance, if a user resource is accessed often and/or modified often in a particular section of code, during execution of that particular section of code the current value of a user resource may be stored in a register which is accessed and updated, without concurrent update of any other storage location. A standard debugging program, therefore, would have no way of accessing this user resource during execution of the particular section. Thus a programmer is forced to return to attempting to debug code based on the object code, or to use some other method.